Boxes of Norlevo emergency contraceptive are sold in a pharmacy, west of Paris, Tuesday Nov. 26, 2013. Health Canada says it is studying the effectiveness of the so-called "morning after" contraceptive pills in the wake of news they don't as well in larger women. THE CANADIAN PRESS/AP/Remy de la Mauviniere

TORONTO - Emergency contraceptive pills known as the “morning-after” pill will soon carry new warnings about reduced effectiveness in women weighing more than 165 pounds.

Health Canada has asked companies to add warnings to packaging advising that the pills are less effective in women weighing 165 to 176 pounds and are not effective in women weighing more than that.

Women who weigh 165 pounds or more are advised to ask a health professional for advice on alternative methods of emergency contraception.

The emergency contraceptive pills available without prescription in Canada are Next Choice, Norlevo, Option 2 and Plan B.

The drugs are taken by women up to 72 hours after unprotected sex or a contraceptive accident, such as a condom breaking, to prevent pregnancy.

The pills contain a higher dose of levonorgestrel compared to regular birth control pills with this hormone and work by preventing ovulation or fertilization of an egg. They have no effect if a woman is already pregnant.